Fail if any polygon points for rotate_extrude are less than 0

A 360 degree rotate extrude with any negative points will most likely cause overlaps (except for an oddly-formed polygon), and not having negative points seems to be implied by line 458 (set max_x to be initially 0, rather than the x location of the first point). This modification makes this explicit, causing a failure if negative points are detected.

It would also be possible to include a min_x variable, and check if it equals 0 -- the documentation suggests this should always be the case [http://en.wikibooks.org/wiki/OpenSCAD_User_Manual/2D_to_3D_Extrusion#Rotate_Extrude].